# CVE-2026-42533: F5 NGINX heap buffer overflow in map directive

Source: https://junglewise.ai/threats/cve-2026-42533-f5-nginx-heap-buffer-overflow-in-map-directive  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-15. Updated: 2026-07-15  
License: CC BY 4.0 with a link back

CVE-2026-42533 · Severity: high · CVSS 8.1 · Published 2026-07-15

NGINX Plus and NGINX Open Source are widely used web servers and load balancers. A vulnerability in how these servers process specific configuration rules could allow an unauthenticated attacker to crash the server or potentially take control of the system. This could lead to website downtime or the theft of sensitive data handled by the server.

Technologies: [F5 Nginx Plus](https://junglewise.ai/threats/technologies/nginx-plus). Vendors: [F5](https://junglewise.ai/threats/vendors/f5).

## Quick answers

**Is CVE-2026-42533 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-42533 being exploited in the wild.

**How severe is CVE-2026-42533?** High severity, with a CVSS score of 8.1, published 15 July 2026.

**Which products and versions are affected?** F5 Nginx Plus R33, R36 before R36 P7, 37.0.0.1 before 37.0.3.1.

**Is there a fix for CVE-2026-42533?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-42533 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-42533?** Check whether you run F5 Nginx Plus at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A heap-based buffer overflow (CWE-122) exists in NGINX Plus and NGINX Open Source when the 'map' directive uses regular expression matching. The vulnerability is triggered when a string expression references the map's regex capture variables before referencing the map output variable, or when using non-cacheable variables under specific conditions. An unauthenticated remote attacker can exploit this by sending specially crafted HTTP requests. Successful exploitation can cause the NGINX worker process to crash (DoS) or allow for arbitrary code execution, particularly on systems where ASLR is disabled or can be bypassed. The issue is confined to the data plane.

## Affected products

- F5 NGINX Plus R33, R36 before R36 P7, 37.0.0.1 before 37.0.3.1
- F5 NGINX Open Source 0.9.6 before 1.30.4, 1.31.2 before 1.31.3

## Timeline

- 2026-07-15: advisory: F5 published the security advisory K000162097
- 2026-07-15: disclosed: CVE-2026-42533 published to NVD

## References

- [https://my.f5.com/manage/s/article/K000162097](https://my.f5.com/manage/s/article/K000162097)

## Related threats

- [NGINX uninitialized memory access in ngx_http_slice_module](https://junglewise.ai/threats/cve-2026-60005-nginx-uninitialized-memory-access-in-ngx-http-slice-module)
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- [F5 NGINX heap buffer over-read in ngx_http_ssi_module](https://junglewise.ai/threats/cve-2026-56434-f5-nginx-heap-buffer-over-read-in-ngx-http-ssi-module)
- [F5 NGINX heap buffer over-read in ngx_http_charset_module](https://junglewise.ai/threats/cve-2026-48142-f5-nginx-heap-buffer-over-read-in-ngx-http-charset-module)
- [NGINX heap buffer overflow in HTTP/2 and gRPC proxy modules](https://junglewise.ai/threats/cve-2026-42055-nginx-heap-buffer-overflow-in-http-2-and-grpc-proxy-modules)